Method for requesting domain transfer and terminal and server thereof

ABSTRACT

A method, terminal and server for controlling a domain transfer operation, are discussed. According to an embodiment, the method includes determining, by a terminal, whether a session is transferred from a first domain to a second domain, wherein the determining is based on an operator policy and radio conditions, wherein the operator policy is received by the terminal from a network, and wherein the operator policy includes at least one of first information indicating an operator&#39;s preferred domain and second information indicating whether to initiate the domain transfer in a short time.

The present application is a Continuation of co-pending U.S. patentapplication Ser. No. 12/576,964, filed Oct. 9, 2009, which is aContinuation of U.S. patent application Ser. No. 11/702,056 (now U.S.Pat. No. 7,623,522), filed Feb. 5, 2007, and for which claims thepriority benefits of U.S. Provisional Application No. 60/765,212 filedon Feb. 6, 2006 and Korean Patent Applications Nos. 10-2006-0116575 and10-2006-0135622 respectively filed Nov. 23, 2006 and Dec. 27, 2006 inthe Republic of Korea. The entire contents of these applications areherein fully incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a Voice Call Continuity (VCC), and moreparticularly, to requesting a domain transfer in the VCC.

2. Discussion of the Background Art

In general, a Voice Call Continuity (VCC) refers to a type ofapplication, namely, a home IMS (IP Multimedia Subsystem) applicationwhich is capable of transporting voice calls between a CS (CircuitSwitching) domain and an IMS domain. The VCC provides functions of voicecall originations, voice call terminations, a domain selection and adomain transfer from the CS domain to the IMS domain or vice versa.Here, the domain transfer refers to transferring access legs for voicecalls toward a user equipment (UE) (i.e., a terminal) from the CS domainto the IMS domain or vice versa during an active session. The access legdenotes a call control leg between a VCC UE and a Domain TransferFunction (DTF) of an VCC application (server).

Through the domain transfer procedures, a service continuity for one ormore voice calls/sessions is provided between the IMS domain and the CSdomain while the VCC UE maintains the one or more voice calls/sessions.

Generally, a domain transfer for a certain voice call/session from theCS domain to the IMS domain or vice versa is initiated only when a DTFis positioned on a signal path of the voice call/session setup. Forthis, positioning of the DTF on the way of the signal path of the voicecall/session setup is referred to as anchoring in IMS or anchoring.

FIG. 1 illustrates a general architecture of a network for providing aVCC service.

As illustrated in FIG. 1, a VCC UE 10 denotes all types of terminalswhich support the VCC service. The VCC UE can access CS and PS (packetswitching) domains. That is, when accessing the CS domain, the VCC UEuses a UE-CS (not shown) provided therein, whereas the VCC UE uses aUE-IMS (not shown) provided therein when accessing the PS domain.

A VCC application 30 is an application server for providing the VCCservice, and is constituted with entities which perform a series offunctions. The series of functions may include functions required tosetup voice calls toward the VCC UE, and functions required to switch anaccess leg of the VCC UE between the CS domain and the IMS domain whilemaintaining (performing) an active session. For instance, the series offunctions can be a Domain Transfer Function 30 a, a Domain SelectionFunction 30 d, a CS Adaptation Function 30 b, and a CAMEL ServiceApplication 30 c. Detailed capabilities and operations for the series offunctions are described in 3GPP TS 23.206 V1.2.0.

Generally, the CS domain entities include a Visited Mobile SwitchingCenter (VMSC), a Gateway MSC (GMSC), a gsmSCF, and the like. The IMSdomain entities include a P-CSCF, a S-CSCF, a I-CSCF and a Media GatewayControl Function (MGCF).

FIG. 2 is a signal flowchart among each component of a network (e.g.,the network shown in FIG. 1) in a case where a domain transfer occursbetween the IMS domain and CS domain. Hereinafter, domain transferprocedures in a VCC service according to a background art will beexplained with reference to FIG. 2.

As shown in FIG. 2, it is just a VCC UE which initiates a domaintransfer in a VCC. That is, the VCC UE establishes a voice call (orsession) over the CS domain or IMS domain with a network, and then maydecide whether or not to initiate a domain transfer by moving (ortransferring) from one domain to another domain. By doing this, theestablished voice call (so-called “ongoing call”) may continue on theway of handing over (i.e., domain transfer) from the CS domain to theIMS domain or vice verse. Here, for initiating (performing) a domaintransfer for the voice call from one domain (e.g., IMS domain) toanother domain (e.g., CS domain), the VCC UE may initiate the domaintransfer based on previously-stored information. Here, the informationpreviously stored in the VCC UE may include radio conditions of a CSnetwork to access, operator policy, user preferences, and the like.

Procedures (1)˜(4) of initiating, by the VCC UE, a domain transfer froma CS domain to an IMS domain according to the background art are asfollows:

-   -   (1) The VCC UE may decide to initiate a domain transfer for a        voice call which has been originated and is ongoing toward the        CS domain, namely, an outgoing call.    -   (2) When the VCC UE sends an INVITE message to a VCC        application, a Domain Transfer Function (DTF) in the VCC        application establishes an IMS session leg for the voice call        over the IMS domain.    -   (3) After the IMS session leg is established between the DTF and        the IMS domain, the DTF routs the ongoing session to the IMS        domain.    -   (4) After routing the ongoing session to the IMS domain, a CS        session leg is released.

Thus, the domain transfer for the voice call is initiated from the CSdomain to the IMS domain through the procedures of (1)˜(4).

For the domain transfer in the VCC, only the VCC UE can initiate thedomain transfer in the current VCC by taking into account certaincriteria (e.g., user preferences, operator policies, etc.). Therefore,according to the background art, there is no way for the network (e.g.,VCC application) to initiate a domain transfer based upon a currentnetwork state, a data load of a specific node, a sustainable QoS(Quality of Service), or the location of the VCC UE.

Further, the network may want to reduce traffic or stop the flow oftraffic over a specific domain during a certain time duration formaintenance or upgrading of the network. In this case, according to thebackground art, there is no method for the network to allow (or request)the VCC UE to initiate the domain transfer for a current call over aspecific domain (i.e., over the CS domain or IMS domain).

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to request, by anetwork (e.g., VCC application). a terminal (e.g., VCC UE) to initiate adomain transfer according to a network state or other reasons.

It is another object of the present invention to provide a method anddevice for requesting a domain transfer, which address the limitationsand disadvantages associated with the background art

To achieve these and other objects according to the present invention,there is provided according to an aspect of the present invention amethod for requesting a domain transfer to be initiated, comprising:evaluating domain transfer request information received in a networkserver; transferring, by the network server, a domain transfer requestmessage to the terminal based upon the evaluation of the domain transferrequest information; receiving and evaluating the domain transferrequest message by the terminal; and deciding, by the terminal, whetherto initiate the domain transfer for a call based upon the evaluateddomain transfer request message.

To achieve these and other objects of the present invention, there isprovided according to another aspect of the present invention a terminal(or UE) comprising: a receiver for receiving a domain transfer requestmessage from a network server; a transmitter for sending a responsemessage with respect to the received domain transfer request message;and a VCC enabler for deciding whether or not to initiate a domaintransfer for an ongoing call by evaluating the domain transfer requestmessage, and then initiating the domain transfer according to thedecision.

To achieve these and other objects of the present invention, there isprovided according to another aspect of the present invention a networkserver comprising: a receiver for receiving a domain transfer requestinformation and also receiving a response message transferred by aterminal; an evaluating unit for evaluating the received domain transferrequest information by comparing it with previously stored conditions; amessage generator for generating a domain transfer request message basedupon the evaluated domain transfer request information; and atransmitter for sending the generated domain transfer request message tothe terminal.

According to another aspect, the present invention provides a terminalfor controlling a domain transfer operation, comprising: a receiver toreceive a message from a network server, the message including domaintransfer related information; and a controller to evaluate the domaintransfer related information and at least one of user preferenceinformation and operator policy information, to determine whether toinitiate a domain transfer of a call based on the evaluation result, andto selectively initiate the domain transfer based on the determinationresult.

According to another aspect, the present invention provides a terminalfor controlling a voice call continuity (VCC) operation, comprising: areceiver to receive a message from a network server, the messageincluding domain transfer related information; and a controller toevaluate the message including the domain transfer related information,to determine whether to initiate a domain transfer of a call based onthe evaluation result, and to selectively initiate the domain transferbased on the determination result.

According to another aspect, the present invention provides a networkdevice for controlling a domain transfer operation, comprising: acontroller to analyze domain transfer request information based oncriteria information, and to generate a domain transfer request messagebased on the analysis result; and a transmitter to transmit the domaintransfer request message to at least one terminal, for controlling theterminal to selectively initiate a domain transfer of an ongoing call.

According to another aspect, the present invention provides a server forcontrolling a domain transfer operation, comprising: a receiver toreceive a domain transfer request; a controller to evaluate the domaintransfer request based on criteria information, to determine whether ornot to generate a domain transfer request message based on theevaluation result, and to generate the domain transfer request messagebased on the determination result; and a transmitter to transmit thedomain transfer request message to at least one terminal so as toselectively initiate a domain transfer for a call.

According to another aspect, the present invention provides a method ofcontrolling a domain transfer operation, comprising: determining, by anetwork server, whether or not to request an initiation of a domaintransfer of a call; generating, by the network server, a message basedon the determination result, the message including a request to aterminal for an initiation of a domain transfer of a call; andtransmitting, by the network server, the message to at least oneterminal.

According to another aspect, the present invention provides a method forcontrolling a domain transfer operation, comprising: receiving, by aterminal, a domain transfer request message from a network server, thedomain transfer request message including domain transfer relatedinformation; evaluating, by the terminal, the domain transfer relatedinformation and at least one of user preference information and operatorpolicy information; determining, by the terminal, whether to initiate adomain transfer of an ongoing call based on the evaluation result; andselectively initiating, by the terminal, the domain transfer of theongoing call based on the determination result.

These and other objects of the present application will become morereadily apparent from the detailed description given hereinafter.However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of theinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention willbecome apparent to those skilled in the art from this detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention.

FIG. 1 is a network architecture for providing a VCC service, in whichthe present invention can be implemented.

FIG. 2 is a signal flowchart among components of a network for a domaintransfer between an IMS domain and a CS domain according to a backgroundart.

FIG. 3 is a signal flowchart between a UE and a VCC application forillustrating a method for generating and transmitting a domain transferrequest in accordance with one embodiment of the present invention.

FIG. 4 is a block diagram of a terminal or UE according to an embodimentof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is applied to a Voice Call Continuity (VCC) in3GPP (3rd Generation Partnership Project), but it may be applicable toother communication fields.

Substantially in the present invention, first, a network server (e.g.,VCC application server) transfers to one or more UEs (i.e., terminal(s))a message which includes information related to a domain transferrequest (this information is also referred to as domain transfer relatedinformation). The domain transfer related information can include, butis not limited to, one or more of the following: domain transfer levelinformation (information regarding a level (or class) of a domaintransfer request), network state information (information regarding anetwork state or the like), evaluation information (informationregarding an evaluation (or analysis) of a domain transfer request madeby an external node or internal module of the network server or thelike), etc. The message including the domain transfer relatedinformation can include other information, and can be a message forother purpose.

Second, the UE receives the message including the domain transferrelated information from the network server, and decides whether or notto initiate (perform) the domain transfer with respect to a call (e.g.,an ongoing call or any subsequent call) based upon the information. TheUE according to the present invention decides whether or not to initiatethe domain transfer with respect to the calls based upon the domaintransfer related information alone, or upon the domain transfer relatedinformation with at least one of operator policy and user preferences.However, considering all of the operator policy, user preferences thatthe UE itself has as well as the received domain transfer relatedinformation is preferred.

Hereinafter, terms preferably used to describe the present inventionwill be defined.

A UE according to the present invention may include all types ofterminals which can be used for the VCC service. For instance, the UEaccording to the present invention comprehensively includes mobilecommunications terminals (e.g., UEs, mobile phones, cellular phones, DMBphones, DVB-H phones, PDAs, PTT, etc.), digital TV sets, GPS navigationdevices, portable game machines, MP3, other home appliances, and thelike.

User preferences denotes information (or parameter) for defininginformation related to a domain selection (or domain transfer) whichindicates what domain the UE user prefers to use for ongoing calls(e.g., incoming or outgoing calls) when the UE (or terminal) isavailable over both the CS domain and IMS domain.

Operator policy denotes information related to a network state or anetwork management. The operator policy may frequently be updateddepending on the network state or for the network management. Theupdated operator policy may include information for requesting a domaintransfer for ongoing calls (or subsequent calls), and/or information ona domain preferred by the network for the domain transfer.

An ongoing call denotes a call in progress between a UE and a VCCapplication, which includes all of the incoming or outgoing calls. Thepresent invention is applicable to ongoing calls, but is not limitedthereto and can be applied to subsequent calls.

A ‘V3 interface’ used in the present invention preferably denotes aninterface for a mutual information transfer between a UE and a VCCapplication. For instance, the V3 interface is shown as 20 in FIG. 1 asan example. The V3 interface may be include a Ut interface, and an OMADM (device management). That is, an information transfer from the UE tothe VCC application can be achieved by use of the Ut interface, whereasan information transfer from the VCC application to the UE can beachieved by use of the DM using a DM method. As another example, the V3interface method may include an unstructured supplementary service data(USSD) method. The Ut interface, the DM method and the USSD method areknown in the art. A protocol of the V3 interface may exemplarily be aHTTP, which has a XML format.

Hereinafter, constructions and operations of the preferred embodimentsof the present invention will be explained in detail with reference tothe accompanying drawing.

FIG. 3 is a signal flowchart between a UE 100 and a VCC application 300for requesting a domain transfer for a call in accordance with oneembodiment of the present invention. This method can be implemented inthe network architecture of FIG. 1 or in other suitable architecture.For instance, the UE 100 and the VCC application 200 of FIG. 3 can berespectively the UE 10 and the VCC application 30 of FIG. 1.

Referring to FIG. 3, a call is currently in progress (i.e., ongoingcall) between the UE 100 and the VCC application 300 over the CS domainor IMS domain (S1). Here, it is assumed in FIG. 3 the ongoing call is inprogress over the CS domain. The ongoing call may denote a call whichhas been initiated in the CS domain, but it may also be a call which wasinitiated in the IMS domain and has been transferred to the CS domain.

During the ongoing call in progress between the UE 100 and the VCCapplication 300, the VCC application 300 (i.e., a type of networkserver) receives from an external node (e.g., a node that controlsoperator policy, a node that controls mobility management, etc.) arequest (or signal, information, or the like) for requesting a domaintransfer from a current domain (e.g., the CS domain) to another domain(e.g., the IMS domain) with respect to a call placed by a specific user(i.e., a user of the UE) (S2). As a variation, the VCC application 30can receive this request from an internal module (e.g., 30 a, 30 b, 30c, 30 d, or other in FIG. 1) of the VCC application 300.

The domain transfer request made by the network (e.g., from the externalnode or internal module) in step S2 may be made by several reasons, forexample, maintenance of a network, traffic load of a specific node inthe network, control related to operator policy, or the like. The domaintransfer request may be specifically made to apply to one (single)terminal which a certain VCC subscriber is using, or to a certain group(e.g., other VCC subscribers) of terminals, or may be non-specificallymade to apply to multiple subscribers (e.g., terminals used by themultiple subscribers).

In step S2, as mentioned above, the domain transfer request may not onlybe transmitted from the network (i.e., the external node) to the VCCapplication 300, but can also be made (triggered) based upon a specificinternal state of the VCC application 300. For instance, in order toregulate or maintain a traffic with respect to the VCC application 300,or perform the operator policy related to the domain transfer, forexample, a certain entity or module)(e.g., a domain transfer functionmodule, a domain selection function module, etc.) provided in the VCCapplication 300 may transmit a domain transfer request to a processingunit (e.g., a controller) constructed in the VCC application 300 forprocessing the domain transfer request signal. When the VCC application300 receives the domain transfer request (or signal) from the network(e.g., the external node) or its internal component (e.g., a certainentity therein), the VCC application 300 performs an analysis orevaluation of the domain transfer request (S3). Information obtainedfrom this evaluation can be referred to as evaluation information, whichcan be included in the message sent from the VCC application to the UEin S4 to be discussed later.

In Step S3, for instance, the VCC application 300 evaluates the domaintransfer request under a previously defined condition, e.g., radioattachment state of the terminal, media type of the ongoing call, etc.(i.e., it checks whether the requested domain transfer is appropriatefor the ongoing call of the UE 300 from the VCC application side). Here,the previously defined condition denotes information related to thedomain transfer, the information stored in a certain entity provided inthe VCC application or a certain external entity. In other words, theVCC application 300 analyzes or evaluates whether or not a messageincluding domain transfer related information should be generated and/ortransmitted to the UE according to the domain transfer request. Here,the domain transfer request may be specific for a terminal which acertain subscriber is using or for terminals which a certain group ofsubscribers are using.

Then in step S4, the message including the domain transfer relatedinformation is sent from the VCC application 300 to one or more UEs 100.The domain transfer related information includes domain transfer levelinformation, and can further include at least one of network stateinformation and evaluation information. The domain transfer relatedinformation is now discussed in more detail.

In step S3, when the VCC application 300 evaluates the domain transferrequest, it decides (or evaluates) what level (or class) the domaintransfer request should be assigned to and generates the domain transferlevel information indicating the level of domain transfer request. Thelevels of the domain transfer request, for example, may be classifiedinto first to third levels (or classes). Examples of these levels(domain transfer level information) can be as follows, but the inventionis not limited thereto:

-   -   A first level of domain transfer request, which is so-called        “shall be” (must be), is to request that the domain transfer        shall be (must be) performed (e.g., if not, the call will be        disconnected in a short time);    -   A second level of domain transfer request, which is so-called        “should be” (should preferably be), is to request that the        domain transfer should be performed (e.g., if not, the call will        risk being disconnected due to resources shortage, etc.); and    -   A third level of domain transfer request, which is so-called        “requested” (can be), is to request the domain transfer be        performed by a selection of the UE based upon a traffic load,        for example (e.g., for load balancing, but up to the UE).

These levels of the domain transfer request can be represented by usingdifferent values of a parameter for indicating the levels of the domaintransfer request, by using different parameters corresponding todifferent levels, by using a type of indicator, or by using other ways.

Accordingly, the domain transfer related information sent from the VCCapplication 300 to the UE(s) in step S4 includes the domain transferlevel information which indicates a level of domain transfer request,which can be, e.g., one of the first to third levels discussed above.

As a variation, the message sent from the VCC application to the UE(s)in step S4 can include a request to initiate a domain transfer, withoutincluding the above domain transfer level information. Also, one ofdifferent or dedicated messages corresponding respectively to differentlevels of the domain transfer request can be sent from the VCCapplication server to the UE(s). For instance, there may be messagesdedicated to indicate different levels of a domain transfer request.Also, there may be other levels of the domain transfer request, for thedomain transfer level information.

Also as mentioned above, the domain transfer related information (S4)may include associated information regarding the domain transfer requestmade by the network that the VCC application has evaluated in step S3,and information related to the evaluation. That is, the domain transferrelated information can include the evaluation information. Also, thedomain transfer related information can include other information (e.g.,the network state information regarding the network state, etc.).

Also the domain transfer related information or the message includingthe domain transfer related information can be included or combined witha message for other purpose, and/or can share the signaling space withother functionality.

In Step S4, the VCC application 300 transmits the message including thedomain transfer related information to the UE 100, which can occur via aVCC UE-VCC application server reference point, i.e., a V3 interface(e.g., V3 interface 20), or using a unicast method. As a variation, themessage including the domain transfer related information can besimultaneously sent from the VCC application 300 to a plurality of UEs,e.g., via a multicasting method or a broadcasting method.

After receiving the message including the domain transfer relatedinformation from the VCC application 300, the UE 100 (or each UE)evaluates (or analyzes) the information included in the message (S5).The UE 100 then sends a response message (e.g., Ack message) to the VCCapplication 300 in response to the received message (S6). Here, theresponse message may include information indicating the decision of theVCC application (and/or UE) with respect to the domain transfer request.For example, the decision on the domain transfer request may indicate anacceptance or rejection of the domain transfer request made by the VCCapplication. Step S6 may be performed between steps S4 and S5, and canbe an optional step.

The UE 100 selectively performs (initiates) a domain transfer of thecall (e.g., ongoing call) by taking into account the information relatedto the domain transfer request which has been evaluated in step S5 (S7).Here, when deciding whether or not to initiate the domain transfer ofthe call, the UE can take into account, as a factor which affects thedecision, the information related to the domain transfer request (i.e.,the domain transfer related information included in the message, as afirst factor) together with other factors that the UE has alreadycontained. Examples of the other factors can be, but are not limited to,user preferences as a second factor) and operator policy as a thirdfactor). Also, priority (level) among these factors (e.g., the first,second and third factors) for making the decision on the domain transfermay be defined according to a user's setup or an operator's setup. Forinstance, if the first and third factors conflict with each other (e.g.,the first factor indicates that the domain transfer should be initiated,whereas the third factor does not), then the UE 100 may decide to followthe first factor according to the setup.

In one example, in step S5, the UE 100 can decide whether or not toinitiate a domain transfer for the current call based on the domaintransfer related information and at least one of the operator policy anduser preferences.

Meanwhile, the reference point between the UE 100 and the VCCapplication 300 can be implemented by use of interfaces or protocolswhich have been defined, and also be implemented by defining newinterfaces. For example, the reference point may be implemented by useof an IMS-based signaling, or by the combination of other interfaces.

In the above example of FIG. 3, the call is an ongoing call (e.g., anoutgoing or incoming call), but the invention is equally applicable toany subsequent call. For instance, the UE 100 can receive the domaintransfer related information from the VCC application 300 in S4. Thenthe UE 100 can store this information and then evaluate it later for adomain transfer of any subsequent call, as desired.

Hereinafter, constructions and operations of the UE 100 (i.e. aterminal) and the VCC application 300 (i.e., VCC application server)according to an embodiment of the present invention will be explained.

The UE 100 according to the present invention may include a primaryhardware construction which is required to process a message includingdomain transfer related information.

For instance, as shown in FIG. 4, the UE 100 according to the presentinvention comprises a receiver 43 adapted to receive a domain transferrequest message (or a message) including information (domain transferrelated information) related to the domain transfer request from the VCCapplication, a transmitter 41 adapted to send (transmit) a responsemessage (e.g., Ack message) with respect to the received domain transferrequest message, a memory or storage unit 42 for storing the receiveddomain transfer related information and other information such asoperator policy, user preferences, etc., and a VCC enabler 46 adapted toevaluate (or analyze) the domain transfer related information includedin the domain transfer request message (and any other information asdesired) to decide whether or not the domain transfer of an ongoing callshould be initiated (performed), to add (or include) informationobtained by the decision (e.g., acceptance or rejection) to the responsemessage, and to selectively attempt to initiate the domain transferaccording to the decision. Here, the VCC enabler 46 may be a controlleras a type of controlling unit for analyzing the domain transfer relatedinformation in order to attempt to initiate the domain transfer. The UE100 may also include other components such as a Ut interface unit 47, aDM unit 48, an input unit 44, and a display unit 45. All components ofthe UE 100 are operatively coupled.

As aforementioned, the operations and functions of each component of theUE which includes technical features of the present invention have beenexplained. Besides, other basic components of the UE for receiving a VCCservice are obvious to those skilled in the art, and the explanationthereof is thus omitted.

The VCC application 300 according to the present invention comprises areceiver for receiving domain a domain transfer request (transferrequest information) from a network (e.g., an external node) due to achange in operator policy with respect to the domain transfer accordingto the network state or for the network maintenance or for some otherreason, and receiving a response message (i.e., Ack message) transferredfrom the UE 100, an evaluating unit for evaluating (or analyzing) thereceived domain transfer request by comparing it with previously storedconditions, a message generator for generating a message includingdomain transfer related information based upon the evaluated domaintransfer request, and a transmitter for sending the generated message tothe UE(s) 100. Here, since the message including the domain transferrelated information is generated by the evaluating unit and the messagegenerator for the purpose of requesting the domain transfer, theevaluating unit and the message generator may be implemented as a typeof controller which is capable of performing the combined functions ofboth the evaluating unit and the message generator.

The present invention has been explained with reference to embodimentsillustrated in the drawings, which, however, are just exemplary. It willalso be apparent to those skilled in the art that various modificationsand variations can be made in the present invention without departingfrom the spirit or scope of the invention. Thus, it is intended that thepresent invention cover modifications and variations of this inventionprovided they come within the scope of the appended claims and theirequivalents.

As described above, the present invention is advantageous because itallows the network server (e.g., the VCC application) to request thedomain transfer from the UE according to the network state or ifnecessary. In addition, the present invention allows the network serverto supply the UE with relevant information (domain transfer relatedinformation) needed for the UE to make a proper informed decision aboutwhether or not to initiate the domain transfer for its call.

Furthermore, the present invention allows the network server to requestthe domain transfer from the UE according to the network state or forthe network maintenance. Accordingly, the UE does not initiate thedomain transfer for the calls unnecessarily and can initiate the domaintransfer only when the network can perform/complete the domain transfer.As a result, the present invention can effectively prevent deteriorationof Quality of Service (QoS) or wasting of signaling or radio resources.

The invention claimed is:
 1. A method for performing a domain transfer,the method comprising: determining, by a terminal, whether to perform adomain transfer of a session from a first domain to a second domain,wherein the determining is based on an operator policy and radioconditions, wherein the operator policy is received by the terminal froma network, wherein the operator policy includes first informationindicating an operator's preferred domain and second informationindicating a domain transfer level, wherein the domain transfer levelindicates a first domain transfer level or a second domain transferlevel, and wherein the first domain transfer level indicates that thedomain transfer needs to be performed as indicated by the network, andthe second domain transfer level indicates that the domain transfer isperformed based on a decision of the terminal.
 2. The method of claim 1,wherein the domain transfer is performed based on the result of thedetermining.
 3. The method of claim 1, wherein the session is an ongoingsession.
 4. The method of claim 1, wherein the session is related to avoice call.
 5. The method of claim 1, wherein the determining is furtherbased on user preference information.
 6. The method of claim 1, whereinthe first domain is a CS (Circuit Switched) domain, and the seconddomain is a PS (Packet Switched) domain.
 7. The method of claim 6,wherein the PS domain includes an IM (Internet protocol Multimedia)subsystem.
 8. The method of claim 1, wherein the first domain is a PS(Packet Switched) domain, and the second domain is a CS (CircuitSwitched) domain.
 9. The method of claim 8, wherein the PS domainincludes an IM (Internet protocol Multimedia) subsystem.
 10. A terminalfor performing a domain transfer, the terminal comprising: a processorconfigured to determine whether to perform a domain transfer of asession from a first domain to a second domain, wherein the determiningis based on an operator policy and radio conditions, wherein theoperator policy is received by the terminal from a network, wherein theoperator policy includes first information indicating an operator'spreferred domain and second information indicating a domain transferlevel, wherein the domain transfer level indicates a first domaintransfer level or a second domain transfer level, and wherein the firstdomain transfer level indicates that the domain transfer needs to beperformed as indicated by the network, and the second domain transferlevel indicates that the domain transfer is performed based on adecision of the terminal.